Power conversion systems are used to provide AC output power to a load, such as motor drives with an inverter stage driving an AC motor. Active front end (AFE) converters employ a pulse width modulated switching rectifier to convert input AC power and provide DC power to a bus, with the inverter switches converting the DC bus to output currents to drive the load. Such active front end converters are typically coupled with input filters, such as LCL filter circuits connected to each power phase. Since the front end rectifier is a switching circuit, the input filter operates to prevent introduction of unwanted harmonic content into the power grid or other input source. Filter components, including the filter inductors, are typically designed according to the power converter rating, where oversizing input filter components adds cost to the system and occupies valuable enclosure space. However, situations may occur in which grid voltages sag, or in which an available input source voltage is lower than the nominal AC input voltage for which the converter was designed. In certain applications, moreover, it may be desirable to operate a higher voltage motor or other load even though the source voltage is low, for instance, a 400 V input voltage to drive a 460 V motor. In these situations, the active front end rectifier can be operated in boost mode to provide additional boost to increase the gain of the front end converter, thereby boosting the DC bus voltage. At full load conditions, however, boost mode operation of the active front end rectifier leads to increased ripple and other harmonics, which can overheat the filter inductor core. One or more thermal shutoff switches may be positioned to sense the inductor temperature increase and cause a safe system shutdown. However, tripping the drive may not be desired in certain applications, and thus it is desirable to have a technique to allow the system to operate in boost mode without shutdown. In addition, such a thermal switch may be positioned some distance from the inductor core in order to sense temperature increases due to multiple causes, such as to detect whether a system blower fan is off while a full load is being driven, and thus may be unable to quickly detect overheating in the filter inductor core. Adding multiple thermal switches may address this issue, but this approach adds further cost and complexity to the system. In addition to filter inductor overheating issues, active front and rectifiers may also exhibit increased switching loss associated with operation of the rectifier switching devices in a boost mode.
For both of these reasons, operation of an active front end power conversion system in boost mode may require an overall derating of the input and output capabilities of the converter. Specifically, the maximum output current available from the power converter may need to be reduced when the active front end is operated in boost mode in order to mitigate or avoid overheating the filter inductors and/or to reduce rectifier switching losses. However, such derating may render a power conversion system unsuitable for a given application. Accordingly, there is a need for improved power converter apparatus and operating techniques to facilitate operation with an active front end in boost mode while mitigating or avoiding thermal stress to filter inductors and/or rectifier switching losses to achieve improved power ratings.